The present invention relates to an image forming apparatus such as a copying machine or a printer, and particularly relates to a developing device for use in a tandem type image forming apparatus, and an image forming apparatus incorporating such a developing device.
As systems for a developing device incorporated in an image forming apparatus, there are known a two-component developing system using toner and carrier in developer, and a one-component developing system using no carrier but only toner in developer. Of the one-component developing system, as a one-component color developing system, there are known a four-cycle rotary developing system in which developing devices for respective colors rotate to intermittently abut against a photoconductor in order to perform development on the photoconductor, and a tandem developing system in which development is carried out on photoconductors for respective colors with developing devices substantially fixed.
FIG. 10 shows an example of a related-art developing device using such a one-component developing system disclosed in Japanese Patent Publication No. 2001-51497A. As shown in FIG. 10, this developing device 200 has an agitator 203 rotating in a direction to supply toner to a supply roller 201 from below. Each of blade members 209 fixed to a forward end of an arm portion 205 of the agitator 203 scoops up toner 207 substantially to the height of the supply roller 201 so as to guide the toner 207. Thus, the toner 207 guided by the blade member 209 of the agitator 203 is supplied onto a toner guide member 211.
In addition, the toner 207 supplied onto the toner guide member 211 is carried on the circumferential surface of the supply roller 201, and transferred to a photoconductor drum 215 through a developing roller 213. Then, a regulation blade 217 abuts against the circumferential surface of the developing roller 213 so as to scrape excess toner from the circumferential surface down to an area 219 under the developing roller 213. In addition, in the related art, the width of the blade member 209 is smaller than the width of the toner guide member 211 and the width of the supply roller 201.
In the example shown in FIG. 10, the position where the regulation blade 217 abuts against the circumferential surface of the developing roller 213 is substantially as high as or lower than the top surface of the received toner 207. Accordingly, the undersurface side of the developing roller 213 is always in contact with the toner 207. Therefore, the function that the regulation blade 217 scrapes excess toner from the developing roller 213 to thereby control the volume of toner to be conveyed to a developing area (the portion where the developing roller 213 and the photoconductor drum 215 face each other) and the function that the regulation blade 217 charges toner properly are blocked.
In addition, in the example shown in FIG. 10, it is necessary to provide a return roller 223 for circulating the toner 207 scraped by the regulation blade 217 toward a toner receiving portion 221 suffering an agitating action. The structure becomes more complicated and the cost increases for the necessity of the return roller 223.
When a member for returning toner to the toner receiving portion such as the return roller 223 is provided, the toner suffers mechanical stress so that the lifetime of the toner is shortened. In addition, according to a system in which a developing device is fixed, such as the tandem system, it is necessary to provide a member such as a discharge roller for accelerating the circulation of toner forcibly in order to accommodate the toner in the toner receiving portion efficiently. That results in degradation of the toner in an early stage.
Thus, fogging or solid density changes caused by the image degradation in an early stage are so conspicuous as to be a significant factor in reduction of image quality. In addition, fogging increases the toner consumption so that the running cost for expandable supplies increases
Furthermore, the width of each blade member 209 is smaller than the width of the toner guide member 211 and the width of the supply roller 201. Therefore, on the both side end portions of the toner guide member 211 and the supply roller 201, there are areas where the toner is not delivered from the blade member 209. As a result, in the opposite end portions of the supply roller 201, there is a probability that printing is impossible or printing unevenness is caused by flowing-out of the toner from the inner area.
Further, in the example shown in FIG. 10, the width of the toner guide member 211 is set regardless of the width of the supply roller 201. However, when the width of the toner guide member 211 is larger than the width of the supply roller 201, there is excess toner in the opposite ends of the supply roller 201. This excess toner may cause a print in which the printing density is high in the opposite ends of paper. On the contrary, when the width of the toner guide member 211 is smaller than the width of the supply roller 201, toner cannot be supplied all over the effective width of the supply roller 201. This may cause another problem in terms of the relationship to paper that a print low in density in the opposite ends of the paper is made.
Further, in the example shown in FIG. 10, the top surface of the toner guide member 211 indeed has a portion approaching the circumferential surface of the supply roller 201, but even the portion which is closest to the circumferential surface of the supply roller 201 has a distance therefrom large enough for toner to fall through the gap between the toner guide member 211 and the circumferential surface of the supply roller. Accordingly, the reliability with which the toner is carried on the circumferential surface of the supply roller 201 is low. Thus, in a portion where the toner has fallen out, the toner is carried in patches on the circumferential surface of the supply roller so as to cause printing unevenness in a print.
Further, in the example shown in FIG. 10 even in a mode of low duty printing not required a volume of toner as large as that in a normal printing mode, the agitator 203 rotates in the same manner as in the normal printing mode so as to keep on supplying toner onto the toner guide member 211. Accordingly, the toner supply exceeds the toner consumption. It can be therefore considered that the toner runs over the supply roller 201 so that the toner is conveyed directly to the developing roller.
When such a state occurs, not only does unevenness appear in toner volume on the surface of the developing roller, but the charge condition of the toner is also affected to cause trouble in quality of a print.